While there are some cats that would be stressed by being kept inside, I think to insist on access to the outdoors is to miss a very important point.
Zoos try (if they're any good) to make their animals environment as close to natural as possible, not by making it look real to our eyes, but to the animals'.
An indoor cat needs, besides a litter tray, food and somewhere to sleep, an environment that it finds interesting.
This may be a bit more difficult than for outdoor cats, but it's far from impossible, and there's the benefit of safety. To a cat, cars, for example, are completely unnatural.